1. Field of the Invention
The present invention generally relates to radiation collimators. More particularly, the present invention relates to a focused radiation collimator made from a plurality of groups of identical radiation absorbing layers.
2. Description of the Prior Art
Scattered X-ray radiation (sometimes referred to as secondary or off-axis radiation) is generally a serious problem in the field of radiography because the secondary or off-axis radiation reduces contrast in resulting radiographic images. Accordingly, radiation collimators, usually in the form of grids, are used for a variety of reasons to filter out off-axis radiation from the radiation intended to be observed. Such collimators have been used to filter out off-axis radiation in medical imaging as well as in astronomical observation applications such as X-radiation or gamma-radiation cameras on board orbiting satellites.
Some collimators are made of a radiation absorbing material having an arrangement of slots or channels with pre-specified aspect ratios (depth versus area of opening). Radiation moving in a direction aligned with the channels passes through the collimator substantially unobstructed, while off-axis radiation moving in a direction that is not aligned with the channels is eventually absorbed by the radiation absorbing material forming the collimator body. The channels of such collimators may be parallel to each other or may be angled so as to be aimed towards a radiation point source which is at a known distance from the collimator. Collimators with angled channels are often referred to as focused collimators.
U.S. Pat. No. 5,606,589 discloses a radiation collimator, in the form of an air cross grid, for absorbing scattered secondary radiation and improving radiation imaging in general for low energy radiation applications such as mammography. The collimator is formed by stacking and aligning a plurality of very thin radiation absorbing foil sheets together to obtain an overall thickness suitable for the low energy application. Each of the foil sheets has a relatively large plurality of precision open air passages extending there through. The precision openings are obtained by photo etching techniques. The foil sheets are precisely stacked so that the precision openings of the metal foil sheets are aligned. In one embodiment, the openings in each metal foil sheet are formed so as to be progressively increasingly angled relative to the planar surfaces of the foil sheet. This is accomplished by photo-etching the foil sheets from both sides with two slightly different photo-etching tools. For example, in a focused collimator containing 24 metal foil sheets made according to the teachings of this invention, 26 different photo etching tools must be used. The use of a relatively large number of photo etching tools can make the process for making such collimators somewhat expensive. Although, the same manufacturing techniques can be used to make a very high aspect ratio collimator comprising 700 or more foil sheet layers, as the number of unique layers increases, the difficulties of aligning a large number of unique layers so that the precisely etched openings of the collimator will be accurately focused at the radiation point source increases tremendously.